Effect of power on the microstructure and mechanical properties of 1500 MPa martensitic steel joints in laser-assisted friction stir welding

Abstract

This study employs laser-assisted friction stir welding (FSW) to join martensitic steels, aiming to produce defect-free welds. A comparative analysis was conducted to examine how varying laser power influences the evolution mechanisms in different weld zones and enhances joint mechanical properties. The experiments demonstrated successful defect-free welds across a range of laser powers. With increasing laser power, the stir zone (SZ) expanded, and the hierarchical martensitic structure grew progressively larger. At 250 W, the SZ exhibited block tempered martensite due to extended tempering, whereas strip tempered martensite formed at other power levels. The thermo-mechanically affected zone (TMAZ) exhibited a reduction in ferrite content of up to 24.0% as laser power increased, along with increases in the kernel average misorientation (KAM) value and the fraction of deformed grains, which rose by up to 28.3% and 86.2%, respectively. In the heat-affected zone (HAZ), martensite tempering facilitated the precipitation of fine Fe₃C particles, which became more spherical morphology and grew in diameter by up to 59.0%. At 750 W, the FSW joints and SZ achieved the highest ultimate tensile strength (UTS), with increases of up to 8.4% and 9.2%, respectively. These improvements were attributed to minimal ferrite content in the TMAZ and reduced tempering in the SZ.